1. Technical Field
The present disclosure relates to display technology, and more particularly to a method for fabricating a flexible display device.
2. Description of Related Art
In recent years, scientists and engineers have been enticed by the vision of flexible displays. A flexible display defined in this disclosure is a flat-panel display using a thin and flexible substrate, which can be bent to have a radius of curvature of a few centimeters or less without the loss of functionality. Flexible displays are considered to be more attractive than conventional rigid displays, since they allow more freedom in design and promise smaller and more rugged devices.
The flexible thin film transistor (TFT) substrate is one of the most important components of the flexible displays, and on which considerable attention has been focused. Conventionally, flexible substrates, such as a plastic substrate, thin glass substrate or metal foil substrate, have been utilized to provide a mechanical flexibility to display devices. However, during the high temperature process of forming an active device such as TFT directly on a plastic substrate, the plastic substrate having a glass transition temperature of less than 200° C. is subjected to dimensional instability. In addition, the thin glass substrate suffers from several disadvantages such as higher cost and lower impact resistance.
The metal foil substrate provides higher heat resistance, higher solvent resistance, lower thermal expansion coefficient, good flexibility, higher strength, and good gas permeability resistance, such that it is an optimal substrate material for flexible TFT display devices. However, a major problem using the metal foil substrate is higher surface roughness. FIG. 9 is a schematic, partial, cross-section of a conventional metal flexible TFT substrate. The metal flexible TFT substrate 100 includes a metal substrate 102 and an active device 106. However, it is very difficult to form the active device 106 directly on the metal substrate 102 since the metal substrate 102 has reduced surface flatness by having a root mean square (RMS) exceeding 1000 {acute over (Å)}. In addition, some peaks 104 formed on a surface of the metal substrate 102 may be detrimental to the properties of the active device 106. In order to overcome the above described limitation, polishing techniques have been utilized to decrease the surface roughness of the metal substrate 102. Such techniques include chemical mechanical polishing (CMP), electro chemical polishing (ECP), and super mirror methods. In an alternate solution, an insulating buffer layer 108, which can prevent contamination by the particles 110, is formed between the metal substrate 102 and the active device 106.
It should be noted that the buffer layer 108 still presents certain disadvantages. For example, in order to cover all of the peaks 104 and decrease the surface roughness, the buffer layer 108 must normally be formed at a greater thickness (1˜5 mm), which is a time-consuming and costly requirement. Furthermore, as thickness increases, the buffer layer 108 tends to crack due to stress, due to the frequent presence of inorganic materials such as silicon oxide.
Therefore, a method for fabricating a flexible display device is desired to overcome the described limitations.